A dual-purpose plant for producing deep drawing steel sheets and hot-dip galvanized steel sheets is disclosed in published Japanese Pat. Appln. No. 132,437/'78. In the known production plant, a reheating zone and a hot-dip galvanizing apparatus are provided at the outlet side of a secondary cooling zone following an overaging zone of a continuous annealing line and said galvanizing apparatus is movable in the vertical direction so that the plant can be used either for production of cold rolled steel sheets or the production of hot-dip galvanized steel sheets and can be switched back and forth freely between these two modes of operation. This plant, however, has disadvantages in that it includes both a reheating zone and a means for vertically moving the galvanizing apparatus, thus increasing the cost of the plant as well as its operating cost, and in that, since the strip is reheated after the overaging treatment, carbides precipitated in the matrix of the steel dissolve in solid solution again, degrading the mechanical properties as a result thereof. Moreover, since the galvanizing apparatus is provided at the outlet side of a secondary cooling zone, an additional cooling means is required to cool the strip after galvanizing, which also results in increasing the cost of the plant.
A dual-purpose plant using continuous hot-dip galvanizing equipment and production equipment for black plates is disclosed in published Japanese Pat. Appln. No. 19,407/'79. In this plant, a hot-dip galvanizing apparatus and a roundabout passaageway are provided between the primary cooling zone of a continuous annealing line for black plates and the overaging zone thereof. In this plant, however, since the galvanized steel sheets are subjected to overaging, alloy formation occurs in the zinc layer of the steel sheet with iron as the substrate during the overaging, whereby the adhesive strength of the galvanized layer is degraded. Also, in this plant, the overaging temperature is kept low for preventing hearth rolls disposed in the overaging zone from picking up zinc from the zinc layer in the overaging treatment. As a consequence, the effect of the averaging is not sufficient to produce deep drawing galvanized steel sheets of satisfactory quality. Moreover, the plant does not possess an in-line temper rolling means which is indispensable to a continuous annealing line having an overaging zone.
For eliminating these defects in plants wherein the overaging treatment system includes reheating before galvanizing and in plants wherein the overaging is carried out after galvanizing, there has been proposed in published Japanese Pat. Appln. No. 149,129/'76 a process wherein a preliminary overaging is conducted before galvanizing and a further, secondary overaging is conducted after galvanizing. In the proposed process, the overaging before galvanizing, which is a preliminary treatment, is performed over a short period of time and then after finishing the galvanizing, a regular overaging treatment at a proper temperature above 350.degree. C. is conducted. However, with this process, adhesion of zinc to the hearth rolls in the overaging zone during the regular overaging treatment is inevitable and any countermeasure taken to avoid this trouble is certain to considerably increase the cost of the plant.